CI: Update to actions/cache@v4

No functional change, but this upgrade is required, due to the v3 runtime being
deprecated:

> Node.js 16 actions are deprecated. Please update the following actions to use
> Node.js 20: actions/cache@v3. For more information see:
> https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/aws-lc.yml

@@ -24,7 +24,7 @@ jobs:
           echo "result=$result" >> $GITHUB_OUTPUT
       - name: Cache AWS-LC
         id: cache_aws_lc
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: '~/opt/'
           key: ssl-${{ steps.get_aws_lc_release.outputs.result }}-Ubuntu-latest-gcc

.github/workflows/vtest.yml

@@ -60,15 +60,15 @@ jobs:
     - name: Cache SSL libs
       if: ${{ matrix.ssl && matrix.ssl != 'stock' && matrix.ssl != 'BORINGSSL=yes' && matrix.ssl != 'QUICTLS=yes' }}
       id: cache_ssl
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: '~/opt/'
         key: ssl-${{ steps.generate-cache-key.outputs.key }}
 
     - name: Cache OpenTracing
       if: ${{ contains(matrix.FLAGS, 'USE_OT=1') }}
       id: cache_ot
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: '~/opt-ot/'
         key: ot-${{ matrix.CC }}-${{ env.OT_CPP_VERSION }}-${{ contains(matrix.name, 'ASAN') }}